It is an important catalytic reaction process that ethylene reacts with oxygen over a metal silver catalyst to form ethylene oxide. The used silver catalyst can be a metal simple substance of silver itself, or a supported silver catalyst. Currently, most of the silver catalysts used in the industry are silver catalysts supported on alumina. For further improving the efficiency of the metal silver catalyst, enhancing the activity, the selectivity and the stability of the catalyst, and improving the economy of the production, many companies and research institutes have continuously made an exploration in the support, the promoter system, the preparation technology, the reaction condition and the like.
It is not economical to use the clumpy metal silver as catalyst in the industry. In addition, the clumpy metal silver has an inferior catalytic performance. Therefore, in many circumstances, the supported silver catalysts, into which other promoters are added, are used to improve the catalytic performance and the economy of the catalyst. Alkali metals and alkaline-earth metals are often used as promoters.
U.S. Pat. No. 4,039,561 disclosed a silver catalyst supported on alpha-alumina, which catalyst, besides silver, contains alkali metals K and Cs, Tl and at least one element selected from Group IVA metal elements, and has a selectivity of only about 78%.
U.S. Pat. No. 4,207,210 issued to Shell Oil Company in 1980 disclosed a silver catalyst supported on alpha-alumina, which catalyst contains one or more of alkali metals K, Rb and Cs, wherein the alkali metal promoter can be impregnated in advance or simultaneously together with silver. The obtained catalyst has a selectivity of up to about 80%.
U.S. Pat. No. 4,305,844 issued to Huels Chemische Werke AG in 1981) disclosed a silver catalyst containing 0.01-0.25% barium and alkali metal promoters, and having low initial activity and selectivity.
U.S. Pat. No. 4,761,394 issued to Shell Oil Company in 1988 disclosed a silver catalyst supported on a porous and heat-resistant support and containing alkali metals and rhenium aid. The catalyst was said to have an improved selectivity.
U.S. Pat. No. 4,766,105 issued to Shell Oil Company in 1988 further disclosed a silver catalyst supported on a porous and refractory support and containing alkali metal promoters, a rhenium promoter and a co-promoter of the rhenium promoter. The catalyst is said to have a further improved selectivity.
It is noticeable that while these catalysts containing rhenium have improved the initial selectivity, the activity and the stability are remarkably decreased.
The support is an important constituent of the supported catalyst. In the industrial silver catalyst, a support mainly composed of alpha-alumina is used for supporting silver. A suitable support should not only have a relative high mechanical strength, but also be capable of providing adequate specific surface area and pore structure. A prepared support can be surface-modified by treating the support to improve the dispersion of the catalytically active ingredient silver, alter the level of acid and base on the support surface, and adjust the surface electron state of metal silver and the adsorption and desorption properties of the reactants, so as to improve the catalytic performance of the silver catalyst. Some research works had been reported in these aspects.
Chinese Patent No. CN1044416 disclosed a silica surface-modified support obtained by coating 0.03-20 wt % of amorphous silica on the surface of the support pores by impregnation, drying and calcining at a high temperature of 700-1500° C. It is said that the catalyst obtained by depositing silver and promoters on the support and heat-treating at a temperature of 400-950° C. in an inert atmosphere has a higher selectivity, a higher activity and a longer lifetime.
U.S. Pat. No. 4,786,743 also disclosed a silver catalyst made from a support which is surface-treated by an acid of pKa≦3 and has a step-like structure. It is said that the catalyst has a relative good thermal stability, activity and selectivity. In this invention, an acid of pKa≦3, such as nitric acid, hydrochloric acid, sulfuric acid, phosphoric acid and oxalic acid is used to treat an alpha-alumina support. Then a formaldehyde solution is used to reduce silver to produce the silver catalyst. The selectivity is at most only about 80%.
DE 2933950 disclosed a catalyst for producing ethylene oxide from ethylene and oxygen. In said catalyst, alpha-alumina is used as the support material, and silver is used as active substance. Said catalyst is characterized in that the Si compound dissolvable in an alkali has a content of lower than 0.001 wt % calculated on SiO2 in said catalyst. It is stated in this patent that a pure α-Al2O3 should be used as the support material. If this pure α-Al2O3 is not commercially available, a purification operation can be started from commonly commercially available α-Al2O3, which is boiled with a substantially equivalent amount of a NaOH solution with a concentration of 1 wt % for 30 minutes, and washed with water free of salt until the solution has a pH of below 8. With this operation, α-Al2O3 can be obtained, wherein the Si compound dissolvable in an alkali has a maximum content of lower than 0.001 wt % calculated on SiO2 in said catalyst. It is also stated in this patent that other methods can be used to decrease the content of the Si compound in the support to lower than 1 ppm. For example, the support can be boiled with a substantially equivalent amount of a hydrofluoric acid solution with a concentration of 1 wt % for 10 minutes, and then washed with water until the pH is greater than 5. It is said that the obtained silver catalyst containing the alkali metal promoter has a remarkably improved activity and lifetime (stability). The supported silver catalyst obtained with this patent has a maximum selectivity of only about 81.7% after two days. The treatment for the silver catalyst is complex.
JP 2002136868 disclosed an alumina support. A support starting material is shaped, and calcined at a temperature of 500-2000° C. to a support precursor which is mainly composed of alumina; the support precursor is treated with a hydrofluoric acid solution having a concentration of 0.1-15%, and calcined at a temperature of 1000-2000° C. for several hours to give the final support. It is said that a rhenium-containing silver catalyst made from this support has a good catalytic activity, selectivity, and lifetime. According to this patent, before shaping, to the support starting material is added alkali metal rather than alkaline-earth metal. In the examples of this patent, a hydrofluoric acid with high concentration such as about 5% or 4% is used to treat the support, and then a calcining treatment at a high temperature of 1000-2000° C. is required. Therefore, the preparation process has a high requirement on the equipment and a high production cost, and it is difficult for the large-scale practice.
Moreover, after the shaped pure α-alumina support is treated by the hydrofluoric acid solution, the mechanical strength remarkably decreases, which can cause the catalyst to break up and fragmentize, resulting in a tail burning in the ethylene epoxidation reactor and a safety problem in the silver catalyst application.